1. Field of the Invention
The present invention relates to an axle driving apparatus using a hydrostatic transmission for individually driving left and right axles. The hydrostatic transmission includes a pair of hydraulic pumps and a pair of hydraulic motors.
2. Related Art
A driver of a vehicle during use needs to have the ability to change the traveling direction of the vehicle, for example, from straight forward travel to a gentle turn, a sharp turn and zero turn. To accomplish this, often a pair of high performance hydrostatic transmissions are used. Such hydrostatic transmissions are individually connected to left and right drive wheels of the vehicle.
U.S. Pat. Nos. 4,920,733 and 4,967,543 disclose examples of such, whereby a pair of variable displacement hydraulic pumps and a pair of fixed displacement hydraulic motors are arranged on a car body. The pair of hydraulic pumps are connected to a prime mover. The pair of hydraulic motors are individually connected to the left and right drive wheels. One of the hydraulic pumps is fluidly coupled with one of the hydraulic motors through a pipe. The other hydraulic pump is also fluidly connected to the other hydraulic motor through a pipe. This type of arrangement requires the use of many pipes, which lowers assembly and production efficiency.
An improvement to the technology discussed above is disclosed in U.S. Pat. Nos. 4,809,796, which has the same assignee as the present application and 5,127,215. These patents disclose using a pair of commercial hydraulic transmissions having an integrated hydraulic pump and hydraulic motor disposed on a transaxle mounted to the left and right sides of a vehicle. The transmissions are coupled in a driving manner with the axles to constitute an axle driving apparatus. Because two commercial hydrostatic transmissions must be used, the manufacturing costs are high.
To overcome the problems discussed above, Japanese Utility Model Laid-Open Gazette No. Hei 2-135,765, also having the same assignee as the present application, discloses an axle driving apparatus which has a pair of variable displacement pumps and a pair of fixed displacement motors in a single housing. The pair of motors are individually connected to the left and right axles, respectively. FIG. 1 of this Japanese document shows the pair of hydraulic pumps and hydraulic motors mounted on a center section disposed in the housing.
The disclosure of this Japanese document relates to controlling the driving of the left and right hydraulic motors when the vehicle is turning. However, the construction disclosed in this document is inadequate in that a commercial device cannot be realized. For example, the construction of the housing, the power transmission routes from the left and right hydraulic motors to the respective axles, the construction of a closed circuit for fluidly connecting each hydraulic pump with each hydraulic motor, and the positioning of a charge pump for supplying operating oil to the closed circuit are all inadequate.
The drive control of the left and right hydraulic motors of this Japanese document is constructed with turn levers on the sides of the vehicle. When the turn lever on a particular side is lightly gripped, the hydraulic motor at that side is stopped so as to gently turn the vehicle. When the lever is tightly gripped, the axle at that side is braked hard to sharply turn the vehicle. In contrast to the device disclosed in the Japanese document, the present application generally discloses a pair of left and right hydrostatic transmissions to eliminate having to turn a vehicle by controlling the output rotation at one side of the vehicle.
Finally, the vehicle disclosed in this Japanese document is designed to travel forward when both of the steering levers are released. However, even when an operator leaves the vehicle unattended, the vehicle may continue to travel, which is extremely dangerous.